This invention relates to lubricated metal workpieces, particularly of steel and aluminium, used in the production of press-formed components, and in particular to a method of using such workpieces to make structures of shaped components.
There is current interest in techniques for producing adhesively bonded structures of shaped aluminium components for use in the automotive industry. Such a technique is described for example in EPA 127343. The technique of converting a coil of aluminium metal sheet into a structure of shaped components for use in the automotive industry may typically involve the following steps.
The metal surface is pre-treated to provide a strongly bonded layer thereon which acts as a base for subsequently applied adhesive. PA1 A lubricant is applied to the treated metal coil. The coil may then be stored or transported, with the lubricant serving to protect the treated metal surface, and is cut up into pieces ready for press-forming. PA1 The pieces of metal sheet are press-formed into components of desired shape. This and subsequent operations are all performed on an automobile production line. PA1 Adhesive is applied to selected areas of the shaped components, without first removing the lubricant. PA1 The components are assembled into the shape of the desired structure, and may be spot welded or otherwise fixed to hold the structure together until the adhesive is cured. PA1 The adhesive is cured at elevated temperature. PA1 The metal surfaces of the structure are subjected to an aqueous alkaline cleaner which removes the lubricant PA1 The structure is painted. PA1 d) After the adhesive has been applied and cured, the lubricant must be readily removable by an aqueous alkaline cleaner of the type conventionally used to prepare metal surfaces for painting. PA1 forming pieces of the sheet into components, PA1 bringing the components together in the shape of the desired structure, PA1 and securing the components together by mechanical and/or adhesive means.
Alternatively, the press-formed components may be secured together to form the structure by mechanical means, e.g. by rivets or spot welds, either in addition to or instead of adhesive bonding.
A lubricant for use in such a technique needs to fulfil several requirements:
a) The lubricant must, obviously, have suitable lubricating properties for the press-forming operation. PA0 b) The lubricant should be solid at likely metal storage temperatures in order to prevent stacked sheets from sticking together. Furthermore, a film of lubricant that is liquid or sticky is prone to smear and to pick up dust and dirt. PA0 c) Since it is not practicable in a production line to remove lubricant prior to application of adhesive, the lubricant needs to be compatible with an adhesive if one is to be used. PA0 a) consists essentially of a full ester of a dihydroxy compound with a C.sub.8 -C.sub.18 saturated carboxylic acid in admixture with a minor amount of the mono-ester of said compound optionally in admixture also with at least one long chain carboxylic acid and PA0 b) has a hardness in the range 0.2-10 N/mm at all temperatures in the range 15-30.degree. C., preferably in the range 0.1-10 N/mm at all temperatures in the range 15-35.degree. C. PA0 a) The nominal fatty acid, e.g. lauric acid, is in fact a mixture of saturated long-chain monocarboxylic acids, typically containing more than 30% of acids other than the nominated one. An effect of these contaminating acids is to depress the melting point of the ester. For the purposes of the invention, esters are preferably derived from fatty acids which are at least 80% pure. PA0 b) The ester is derived from a fatty acid mixture which contains ethylenically unsaturated acids. Such impurities make the lubricant less adhesive-compatible and less easy to remove from the metal surface, and are therefore preferably absent or present in amounts below 5% by weight.
The lubricants of EPA 227360 are designed to be useful, not only for the technique described above, but also for other forming and shaping operations performed on a variety of metals.
In one aspect, EPA 227360 provides a lubricating composition for press forming consisting of a lubricant dissolved or dispersed in a volatile liquid medium, wherein the lubricant comprises at least one ester of a polyhydric alcohol having two or three hydroxyl groups of which one or two are esterfied with a long chain carboxylic acid and has a melting point above ambient temperature but low enough to permit removal from a metal surface by an aqueous alkaline cleaner.
EPA 227360 mentions that mixtures of esters may be used and may be advantageous; and that the lubricant may contain a minor proportion up to 50% of one or more other lubricating compounds such as long-chain carboxylic acids. The lubricants exemplified are: diethylene glycol monostearate in solution in xylene; and diethylene glycol distearate in solution in xylene.
Although the lubricants described in EPA 227360 are generally successful at meeting requirements c) and d), they are sometimes less successful at meeting requirements a) and b). It is surprisingly found that lubricants of this kind are ineffective, so far as aluminium forming operations are concerned, at temperatures above their liquidus. For good aluminium lubricating properties, in ester lubricants of this kind, it appears necessary that some component be present in the solid state, so that the lubricant is solid or at least mushy or viscous, at the forming temperature which may be as high as 35.degree. C. or 40.degree. C. or even higher.
It might appear a simple matter to solve this isolated problem by using a different ester with a higher melting point. A difficulty with this strategy is that higher melting esters tend to be relatively hard at low and ambient temperatures, to the extent that they readily spall and flake off metal surface to which they are applied. Metal forming at 15 or 20.degree. C. cannot satisfactorily be performed under conditions where the lubricant flakes off the metal workpiece. For use in various parts of the world, there is a need for a single lubricant system which meets both these high- and low-temperature criteria. It is an object of this invention to meet that need.